On 2013-05-13 10:28:59 -0400, Noah Misch wrote: > Each worker needs to make SnapshotNow visibility decisions coherent with the > master. For sorting, this allows us to look up comparison functions, even > when the current transaction created or modified those functions. This will > also be an essential building block for any parallelism project that consults > user tables. Implementing this means copying the subtransaction stack and the > combocid hash to each worker. For the sake of completeness, we should also > copy the global MVCC snapshot data (sorting probably won't care). It also > means forbidding, while a parallel task is in flight, operations that affect > the transaction state:
Btw, if you assume you can simply copy a snapshot from the normal backend to the worker backend to make visibility decisions in the general case: You're wrong. Unfortunately you need in-memory state to make sense of combocids... Not impossible to solve, but you should be aware of the issue. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
